A low molecular weight transforming growth factor (TGF) was isolated and purified to homogeneity. The amino acid compositions of human malanoma-derived TGF, murine sarcoma growth factor (SGF) and rat TGFs were then determined. In addition to the production of TGFs some human tumor cells release tumor inhibitory factors (TIFs) which inhibit the growth of some human tumors. Different molecular weight species of TFGs have also been detected in the urine of normal, pregnant and tumor-bearing humans. These TGFs can serve as markers with potential clinical applications. TGFs produced by cells transformed by different strains of feline sarcoma virus isolates have been isolated and compared. Membrance receptors on A431 human carcinoma cells have been characterized. There is now evidence that the genes which code for SGF obtained from murine sarcoma virus-transformed cells are of host cell, rather than viral, origin. Rabbit antisera has been produced against purified EGF receptors from A431 cells which block the binding of SGF, suggesting that SGF exerts its effect through the EGF receptor system on fibroblast cells.